Nanopositioners are widely used in many research and industrial areas. In many fields, such as semiconductor fabrication and quality control, precision sub-micrometer- and nanometer-scale assembly and positioning is required. For example, in the semiconductor field, there exists a constant need to inspect the wafers rigorously, and even to inspect some of the tools used to produce the wafers (such as advanced photomasks). The profitability of semiconductor fabricators depends in large measure on their ability to produce huge numbers of chips with essentially zero defects. As a consequence, quality control is a primary concern. With the smaller features being created using the most up-to-date technologies, the semiconductor industry, among many others, requires critical dimension (CD) surface metrology that is faster and more accurate than ever. Providing CD metrology in turn requires the ability to move a probe across a surface to be inspected in the X and Y directions, and to be able to move the probe toward and away from that surface in the Z direction rapidly, repeatedly, precisely, and accurately.
Nanopositioners based upon flexure hinge designs provide this type of precise, fine-scale motion in the X, XY, or XYZ directions. Nanopositioners also have been developed for use in precision machining, optical switching, cell physiology research, and other applications. The material currently used in fabricating the vast majority of such nanopositioners is Al 7075 alloy. Other aluminum alloys may be used in instances requiring compatibility with ultrahigh vacuum environments. Nanopositioners requiring extreme positioning precision (at the expense of speed) may be made from materials having a relatively low coefficient of thermal expansion (CTE), such as invar alloys. Nanopositioners made from the current materials provide modest performance. Significant changes in nanopositioner performance, however, will require changes in flexure design, especially where heavy loads are to be borne by the nanopositioner.
In addition to semiconductor manufacturing, nanopositioners are also used in such fields as medicine, biotechnology, and precision electronic manufacturing. For example, in biotechnology, individual cells can be manipulated using nanopositioning stages. While the objects being positioned using a nanopositioner are often minute (having masses ranging from far less than one (1) gram in mass to perhaps tens or hundreds of grams), other objects requiring precision positioning have a mass greater than 1 kilogram. Positioning these more massive, “heavy-load” objects requires a nanopositioner having more strenuous, robust properties. The positioning devices currently available do not provide the required stiffness and range-of-motion required to perform optimally under these heavy-load conditions. Therefore, an unmet need exists for a nanopositioning stage that can accommodate heavy loads (e.g., 1 kilogram in mass or greater) and position them with accuracy, precision and speed.
As used herein, the term “nanopositioner” denotes a stage movable within fixed limits and degrees of freedom. Referring now to FIGS. 1 and 2, which depict conventional, prior art nanopositioners, a movable stage 10 is fixed within a frame 12 via a plurality of flexural hinges 14 (sometimes referred to herein simply as “flexures”). The flexures function in combination as a motion guidance mechanism, allowing the stage to move in one or more desired directions, while inhibiting motion in all other directions. A complete nanopositioning system also includes an actuator 16 to move the stage, and a sensor 20 to sense the motion and position of the stage. The actuator and sensor are normally operationally linked to a programmable control circuit (not shown) that drives the actuator and interprets the signals generated by the sensor. Often, the actuator 16 is composed of a piezoelectric element. The typical sensor 18 is usually a capacitance sensor. A strain gauge or an inductive sensor can also be used.
To achieve a pure, single-axis motion, the flexures of a nanopositioner must be designed so as to constrain any off-axis movement, while simultaneously providing smooth, unfettered motion in the desired single axis. In short, the entire stage, flexure, actuator, and sensor combination ideally defines an integrated nanopositioning system that transfers motion from the actuator to the stage in a smooth, elastic deflection of flexures that are essentially friction-free in the desired axis of motion, and immobile in all other axes.
In any nanopositioning stage, the resonant frequency of the stage/flexure/frame combination determines the positioning and/or scanning speed, as well as the stability, of the system. To create a nanopositioning stage that can accommodate heavy loads, the stiffness of the stage must be increased to accommodate the increased load. To increase the stiffness of a stage, the flexure guidance mechanism must be changed to provide smooth motion in the desired axis or axes, while limiting motion in the undesired axis or axes.
Conventional flexure designs have a low stiffness in the moving axis to allow smooth freedom of motion. Low stiffness in the moving axis is also important because it allows the actuator to generate the required translation of the stage while using a minimum amount of driving force. This helps to maintain the accuracy and precision of the stage positioning because a high driving force tends to introduce unnecessary deformations in the frame. If the driving force of the actuator induces frame deformation, the ability of the device to accurately and precisely position the stage is degraded. Conventional flexures also have a high stiffness in other axes to guide the motion of the flexure in the desired axis while limiting motion in all other axes. The stiffness of the stage is controlled by the stiffness of the actuator 16 in the axis of translation. In all other directions, the stiffness of the stage is controlled by the flexures 14. Ideally, the vibration mode of the first resonant frequency is along the translation axis, so that the excited vibration can be suppressed with the actuator and the controller. For a normal load stage, the various conventional, single flexure designs can maintain the stiffness in the non-translation axes at a high level, which maintains the resonant frequencies at sufficiently high levels. For a heavy load stage, however, the overall stiffness of the nanopositioner must be significantly increased to maintain the desired resonant frequencies. While the stiffness in the translation axis can be increased by using a stiffer piezo-actuator, the stiffness of the non-translational axes can only be increased by increasing the stiffness of the flexures. Therefore, the design of the flexures becomes an overriding critical concern as the load to be borne by the stage increases.
There are four typical flexure designs for single-axis nanopositioning stages. These four conventional, prior art designs are shown in FIGS. 2 and 2A. They are the single leaf-spring flexure (FIG. 2, reference “a”), the single notch flexure (FIG. 2, reference “b”), the compound leaf spring flexure (FIG. 2, reference “c”) and the compound notch flexure (FIG. 2, reference “d”). All of these designs have been successfully incorporated into nanopositioners that can be purchased in the commercial markets. The notch flexure can also be expanded to different sub-types of notch flexures depending on the geometric features of the notches, such as a circular notch flexure, an elliptical notch flexure, a flat notch flexure, and the like, as shown in FIG. 2A. While the conventional flexure designs depicted in FIGS. 2 and 2A yield nominally acceptable results for small objects, they cannot function to position kilogram-weight objects precisely and rapidly.